Process to reduce the temperature of a hydrogen and carbon monoxide containing gas and heat exchanger for use in said process

ABSTRACT

A process to reduce the temperature of a hydrogen and carbon monoxide containing gas by contacting the gas with a metal alloy surface having a lower temperature than the temperature of the gas, wherein the metal alloy surface has between 0 and 20 wt % iron, between 0 and 5 wt % aluminium, between 0 and 5 wt % silicon, between 20 and 50 wt % chromium and at least 35 wt % nickel, wherein the metal alloy surface maintains it lower temperature than the temperature of the gas by making use of coolant water.

The invention is directed to a process to reduce the temperature of ahydrogen and carbon monoxide containing gas by contacting the gas with ametal alloy surface having a lower temperature than the temperature ofthe gas.

Such a process is described in EP-A-257719. This publication describes avessel comprising tubes through which a hot carbon monoxide and hydrogencontaining gas flows. The gas is reduced in temperature because thetemperature of the tube surface has a reduced temperature relative tothe hot gas. The temperature of the tubes is kept at a lower valuebecause the tubes are submerged in water. In the cooling process waterwill evaporate. By ensuring that fresh water is supplied to the vesselthe temperature of the tubes can be kept at a lower temperature than thehot gas. The tubes are typically made from metal alloys comprisingsubstantially of iron. Iron containing alloys are preferred because oftheir mechanical strength in combination with their relative low cost.Furthermore usage of these alloys makes it possible to manufacture thecomplicated tube structures of an apparatus as disclosed in EP-A-257719.

A disadvantage of the above apparatus is that in use coke will form onthe interior surface of the tubes because part of the carbon monoxidereacts to carbon and carbon dioxide. Furthermore part of the surfacewill erode resulting eventually in an unacceptable low mechanicalintegrity of the tubes. These effects are especially significant whenthe amount of steam in the hot gas is below 50 vol %. Such a hot CO andH₂ containing gas is for example obtained when performing a partialoxidation of natural gas, refinery gas, methane and the like in theabsence of added steam as described in WO-A-9639354.

It is the object of the present invention to provide a process to reducethe temperature of the hot gas wherein the above described problems sucha coke formation and erosion are avoided or at least minimized.

This object is achieved with the following process. Process to reducethe temperature of a hydrogen and carbon monoxide containing gas bycontacting the gas with a metal alloy surface having a lower temperaturethan the temperature of the gas, wherein the metal alloy surfacecomprises between 0 and 20 wt % iron, between 0 and 5 wt % aluminium,between 0 and 5 wt % silicon, between 20 and 50 wt % chromium and atleast 35 wt % nickel, wherein the metal alloy surface maintains it lowertemperature than the temperature of the gas by making use of coolantwater.

Applicants found that coke formation and erosion can be reduced when theprocess according to the invention is used. Because the alloy layer,which contacts the hot gas, does not comprise a substantial amount ofiron less coke formation and erosion is observed. A preferred supportlayer, which is not in direct contact with the hot gas, provides themechanical strength to the metal alloy surface layer. This isadvantageous because this feature makes it possible to construct forexample the larger diamter tubes as in an apparatus according toEP-A-257719 in an economical manner.

The metal alloy surface layer comprises between 0 and 20 wt % andpreferably between 0 and 7 wt % iron, preferably between 0 and 4 wt %,between 0 and 5 wt % aluminium, between 0 and 5 wt % silicon, between 20and 50 wt % chromium, preferably between 30 and 50 wt % chromium, and atleast 35 wt % nickel. The nickel content balances the total to 100%.

It has been found beneficial to have at least some aluminium and/orsilicon in the metal alloy surface when the concentration of steam inthe hot gas is lower than 50 vol %, preferably lower than 30 vol % andmore preferably lower than 15 vol %. Preferably between 1-5 wt %aluminium and between 1-5 wt % silicon is present in said alloy layerunder such low steam content conditions. The resulting aluminium oxideand silicon oxide layers will provide an improved protection againstcoke formation and erosion when the conditions become more reducing atsuch low steam concentrations. More preferably next to aluminium andsilicon a small amount of titanium and/or REM (reactive elements) areadded to the metal alloy. Examples of REM are Y₂O₃, La₂O₃, CeO₂, ZrO₂and HfO₂. The total content of these added compounds is between 0 and 2wt %.

Preferably the metal alloy surface layer is supported with a metal alloysupport layer having better mechanical properties than said surfacelayer. The metal alloy support layer may be any metal alloy having therequired mechanical strength for a particular application. Typicallythese metal alloys will contain more iron than the surface layer,suitably more than 7 wt % and even up to 98 wt %. Other suitable metals,which can be present in this metal alloy, are chromium, nickel andmolybdenum. Examples of suitable metal allow support layers are carbonsteels and so called low alloy steels having a typical Cr content ofbetween 1-9 wt % and Mo content 0.1-2.25 wt %, austenitic stainlesssteels, for example the AISI 300 series (examples 304, 310, 316) with atypical Cr content of between 18-25% and Ni content of between 8-22%,cast materials, like for example HK-40, HP-40 and HP-modified, nickelbased alloys, for example Inconel 600, Inconel 601, Inconel 690 andIncoloy 800 and ferritic stainless steels, which are Fe based alloyshaving a low nickel content, e.g. less than 2 wt % and a Cr content ofabove 12 wt %.

The two layers of metal alloys may be prepared by methods known to oneskilled in the art. Preferably the metal alloy composite is made bymeans of a building-up welding method resulting in a weld-mountedmulti-layered metal surface. This method is preferred because it enablesone to make difficult tubular structures, as used in the heat exchanger,having the metal alloy surface according to the present invention. Sucha method is characterized in that the desired metal alloy for use as thesurface layer is first atomized by gas atomization to form a powder ofsaid alloy. Preferably the iron content of said powder is substantiallyzero. A layer of the metal alloy is subsequently applied on the supportmetal alloy by built-up welding by plasma powder welding of said powder.After machining the weld metal a flat metal alloy surface is obtained.Thickness of the surface metal alloy may range suitably from 1 to 5 mmand preferably 1 to 3 mm. It has been found that the iron content in themetal alloy layer may contain iron in a situation wherein the startingpowder did not contain iron. This is due to migration of iron from thesupport layer to the surface layer during the welding step. Care shouldbe taken to limit the migration of iron to the surface layer such thatthe end iron content in the surface layer will be below 7 wt % andpreferably below 4 wt %. The iron migration effect can be limited byusing a low iron-content support layer, increasing the layer thicknessand/or by applying the layer in more than one step. A preferred methodto perform such a building-up welding method is described inEP-A-1043084, which publication is hereby incorporated by reference.This publication describes a method to obtain coke resistant furnacereactor tubes for a steam cracker process, which is aimed at preparinglower olefins, e.g. ethylene and propylene.

The gas is suitably obtained in a partial oxidation process of ahydrocarbon feedstock, for example coal, petroleum coke, residualrefinery fractions, bituminous oils, such as ORIMULSION (trademark ofIntevep S.A. Venezuela), natural gas, refinery gas, associated gas or(coal bed) methane and the like. In case a gaseous feedstock likenatural gas is used the partial oxidation is preferably performed in theabsence of significant amounts of added steam, and preferably in theabsence of added steam, as moderator gas. The feedstock to the partialoxidation may also comprise recycle fractions comprising hydrocarbonsand carbon dioxide as may be obtained in downstream processes which usethe CO/H₂ containing gas as feedstock. An example of a suitable partialoxidation process is the so-called Shell Gasification Process asdescribed in the Oil and Gas Journal, Sep. 6, 1971, pp 85-90.Publications describing examples of partial oxidation processes are U.S.Pat. No. 4,132,065, EP-A-291111, WO-A-9722547, WO-A-9639354 andWO-A-9603345.

The temperature of the hydrogen and carbon monoxide containing gas ispreferably reduced from a temperature of between 1000 and 1500° C. to atemperature between 300 and 750° C. The hydrogen to CO molar ratio willdepend on the feedstock of the partial oxidation process. For examplewhen a solid or liquid feedstock is used an H₂ to CO molar ratio of thehot gas is preferably between 0.5 and 1.5. When the feedstock is agaseous feedstock, like for example natural gas, this ratio ispreferably between 1.6 and 2.5.

In case the feedstock of the partial oxidation is a solid hydrocarbonfeed, for example coal or petroleum coke, the reduction of thetemperature of the resulting hot gas according to the process of thepresent invention is preferably performed at the exterior of watercooled tubes as placed in a vessel. Examples of such heat exchangervessels are described in EP-A-342767 and EP-A-722999.

In case the feedstock of the partial oxidation is a liquid or gaseous(at ambient conditions) hydrocarbon feed as described above thereduction of the temperature of the resulting hot gas according to theprocess of the present invention is preferably performed at the interiorof a conduit. In such an embodiment the metal alloy surface maintains itlower temperature than the temperature of the gas in one by contactingthe metal alloy support layer at its free side with coolant water as forexample described in the afore mentioned EP-A-257719. In such anembodiment the gas is cooled by passing the gas through one or moreconduits, which conduits are submerged in coolant water as contained ina vessel in which vessel steam is formed and discharged from said vesseland wherein the interior of the tubes consists of the metal alloysurface layer and the exterior of the tubes consist of the metal alloysupport layer.

The invention is thus also related to a heat exchanger apparatus suitedfor lowering the temperature of a hot gas, comprising a vessel having acompartment for cooling water, an inlet for the gas to be cooled, anoutlet for cooled gas, an outlet for heated steam and a collecting spacefor maintaining generated steam and at least one primary evaporator tubepositioned in the compartment for cooling water and fluidly connected tothe inlet for the gas to be cooled, and at least one steam tube forwithdrawal of generated steam from the collecting space for maintaininggenerated steam and an inlet for fresh water, wherein the interior ofthe primary evaporator tube material consists of the above defined metalalloy surface and the exterior consists of the above defined metal alloysupport layer.

Preferably the above apparatus may suitably comprise at least onesecondary tube-shell heat exchanger vessel, which is used as ‘superheater module’. Said extra vessel is positioned in the compartment forcooling water, such that the generated steam can be further heatedagainst partially cooled gas from the primary evaporator tube. In thisembodiment the term coolant water as used in the claims refers to waterin the gaseous phase having a cooling capacity relative to the partiallycooled gas. An example of such an apparatus is described in theabove-mentioned EP-A-257719. More preferably the primary evaporator tubeis fluidly connected to the tube side of the super heater module and thesteam tube for withdrawal of generated steam is fluidly connected to theshell side of the super heater module.

The invention will be illustrated by the following non-limitingexamples.

EXAMPLE 1

A gas mixture as produced in a partial oxidation of natural gas andhaving the properties as listed in Table 1 was contacted for 4000 hoursand at about 600° C. with a metal alloy surface having the compositionas listed in Table 2. The metal alloy surface was positioned on top of asupport layer by the building-up welding method as described inEP-A-1043084. The support layer was a low alloy steel having theproperties of the metal used in Comparative A (see Table 2). Visualinspection and mechanical inspection showed no deterioration of themetal alloy surface after 4000 hours. Furthermore no coke formation wasobserved.

TABLE 1 CO 33 (mol %) H₂ 55 H₂O 7.5 CO₂ 4.5 Soot 50-100 ppm

TABLE 2 Example 1 2 Comparative A Ni 42 54 Cr 39 25  1.0 Al — 3 Fe 15.516 97.85 (balance) Ti 0.2 0.2 Mo 2.4 0.9  0.6 Mn 0.9 0.9  0.55

EXAMPLE 2

Example 1 was repeated except that a different metal alloy surface wasused (See Table 2). The results were as in Example 1.

COMPARATIVE EXPERIMENT A

Example 1 was repeated except that a different metal alloy surface wasused (See Table 2). In contrast to Examples 1 and 2 heavy coke formationwas observed.

1. A process to reduce the formation of coke and to reduce thetemperature of a hydrogen and carbon monoxide containing gas obtainedfrom the partial oxidation of a hydrocarbon feedstock by contacting thegas with a metal alloy surface having a lower temperature than thetemperature of the gas, wherein the metal alloy surface comprisesbetween 0 and 20 wt % iron, between 1 and 5 wt % aluminium, between 0and 5 wt % silicon, more than 30 wt % chromium and at least 35 wt %nickel, wherein said alloy metal surface is maintained at a lowertemperature than the temperature of the gas by making use of coolantwater, and whereby coke formation on the metal alloy surface isminimized.
 2. The process of claim 1, wherein the metal alloy surfacecomprises between 1 and 5 wt % silicon.
 3. The process of claim 2,wherein the metal alloy surface comprises between 0 and 2 wt % titaniumand/or REM.
 4. The process of claim 3, wherein the metal alloy surfaceis supported with a metal alloy support layer having better mechanicalproperties than said surface layer.
 5. The process of claim 4, whereinthe metal alloy support layer comprises between 7 and 98 wt % iron. 6.The process of claim 5, wherein the metal alloy surface layer is appliedto the metal alloy support layer by means of a build-up welding method.7. The process of claim 6, wherein the temperature of the hydrogen andcarbon monoxide containing gas is reduced from a temperature of between1000 and 1500° C. to a temperature between 300 and 750° C.
 8. Theprocess of claim 7, wherein the hydrogen and carbon monoxide containinggas has a hydrogen to CO ratio of between 1.6 and 2.5.
 9. The process ofclaim 8, wherein the gas comprises less than 15 vol % steam.
 10. Theprocess of claim 9, wherein the hydrocarbon feedstock to the partialoxidation process used to obtain the hydrogen and carbon monoxidecontaining gas is natural gas.
 11. The process of claim 10, wherein thegas is cooled by passing the gas through one or more conduits, whichconduits are submerged in coolant water as contained in a vessel inwhich vessel steam is formed and discharged from said vessel and whereinthe interior of the tubes consists of the metal alloy surface layer andthe exterior of the tubes consist of the metal alloy support layer. 12.The process of claim 1 wherein the metal alloy surface has a thicknessof from 1 to 5 mm and has an iron content less than 7 wt %.
 13. Theprocess of claim of claim 2, wherein the hydrogen and carbon monoxidecontaining gas contains less than 30 vol % steam.
 14. The process ofclaim 12 wherein the metal alloy surface layer has a thickness of from 1to 3 mm and an iron content of less than 4 wt %.
 15. A process to reducethe temperature of a hydrogen and carbon monoxide containing gas bycontacting the gas with a metal alloy surface having a lower temperaturethan the temperature of the gas, wherein the metal alloy surfacecomprises between 0 and 20 wt % iron, between 1 and 5 wt % aluminium,between 1 and 5 wt % silicon, more than 30 wt % chromium and at least 35wt % nickel, wherein said alloy metal surface is maintained at a lowertemperature than the temperature of the gas by making use of coolantwater.